Autonomously self-illuminating or photoluminescent objects are required, first and foremost, in clocks, on bezels, or in other instruments, for example, in the cockpit of aircraft in order to highlight the important points on indicators and labels of the instruments. Thus, the observer is able to read the setting of the instruments even in poor lighting or in darkness. Other examples of applications are sighting aids for weapons (sights). Such self-illuminating devices have no access to a power supply and are often very small. Even larger versions of such self-illuminating or photoluminescent objects are manufactured for other applications. In many countries, emergency exits, light switches, door handles, or other objects or locations, which must be found quickly in the event of a sudden power failure, are marked therewith. In addition, safety personnel identify certain important objects, for example, flashlights, using such self-illuminating markers.
Self-illuminating gaseous tritium light sources (GLTS), in particular, are known. These are closed glass capsules which are internally coated with a phosphor and are filled with the low-level radioactive tritium gas. Substances which can be excited via radiation to illuminate are colloquially referred to as phosphors. This effect is referred to as fluorescence and does not persist or only very briefly persists, for example, for approximately a few milliseconds. Examples of such substances are CRT phosphors, including zinc sulfide and zinc oxide, which glow in the presence of radioactive radiation.
Such radioluminescent capsules glow for decades, due to the long half-life of the tritium gas, and have proven to be highly effective. Since their permanent luminosity is rather weak, however, they are less noticeable in bright conditions, where they appear to be white. At dusk or in darkness, they are perceived by the human eye only after a while, when the eye has become accustomed to the darkness.
Light guides are also known, which collect the ambient light over a large area and release it at a certain, smaller area, whereby this area glows brightly. Disadvantages thereof are the large area which must be exposed to light, and the fact that the light guides do not glow in darkness.
Further known alternatives to luminescence are photoluminescent, so-called phosphorescent paints of the type often found on hands and points on clocks and on bezels. These paints, some of which continue to afterglow strongly and for a long time, are difficult to apply and must be well protected against environmental influences, in particular against moisture.
Document WO 2014/033151 (U.S. Pat. No. 9,488,318, which is hereby incorporated herein in its entirety by this reference for all purposes) provides a method for producing a permanent lamp, a GTLS, of the type mentioned at the outset. For this purpose, an inner wall of a glass hollow body is coated with a fluorescent and/or phosphorescent substance before the cavity is filled with a medium emitting a decaying radiation, and is hermetically sealed. The objective of this method is to cause the substance contained in the cavity to glow by way of the decaying radiation, to which the substance is permanently exposed.
Phosphorescence is generally understood to be the long afterglow of pigments, wherein the term is often confused with phosphor, which is responsible for the fluorescence which does not continue to glow. In the aforementioned document, zinc sulfide, zinc oxide, zinc cadmium, magnesium sulfide, and Y2O2S—all of which are fluorescent and not phosphorescent and, therefore, do not continue to glow or only very briefly continue to glow—are named as examples of such fluorescent and/or phosphorescent substances.
In contrast to radioluminescent substances, which are excited via radioactive radiation, photoluminescent materials are excited via photons, often via UV radiation, in particular. As a result, objects appear brighter in daylight, as is known from highlighters. Their molecules absorb energy from ultraviolet light and emit this energy in the form of visible light; they fluoresce and do not continue to glow.